1. Field of the Invention
The present invention relates to a deaerator for particulates or particulate materials, and more particularly to a deaerator which is adapted to increase bulk density of particles and reduce the volume of the particulates, to facilitate handling operations such as bagging, transportation etc.
2. Description of the Prior Art
A conventional deaerator of such a type is generally constructed as shown in FIGS. 9 to 11. The deaerator includes a cylindrical body 1 with a chargeport or inlet 10 in its upper portion at one end, for charging the particulates and a discharge port or outlet 11 in its lower portion at the other end for discharging the particulates. In the cylindrical body 1, a screw conveyer 4 having a constant pitch screw along its whole length is rotatably supported. The cylindrical body 1 has a perforated cylindrical section 1a at an intermediate portion, to serve as a filter.
Outside of the cylindrical section 1a, an outer cylinder 2 is located concentrically, spaced at a predetermined distance from the cylindrical section 1a. An evacuation pipe 8 is provided for vacuum suction and compressed-air pipes 9 and 9' are provided for back wash of the filter, so that the space between the cylindrical section 1a and the outer cylinder 2 may serve as an evacuation chamber. A filter comprising perforated metal plates 14' and 14" with a filter fabric 13 sandwiched between them is used as the perforated section 1a.
In this converntional deaerator, when particulates which still have a low bulk density are charged through the charge port 10 into the cylindrical body 1, air in the particulates is evacuated through the evacuation pipe 8 by the suction of a vacuum pump, to increase the build density of the particulates while transporting the particulates through the transfer chamber formed between the cylindrical body 1 and the screw conveyor 4. This is achieved by means of the screw conveyor and the particulates, with increased bulk density, are discharged from the discharge port 11.
The particulates transported in the transfer chamber by the screw conveyor 4 are gradually deaerated while being guided through the perforated cylindrical section 1a. Thus, the bulk density gradually increases and the volume of the particulates decreases. However, this concurrently causes the ratio or degree of filling of the particulates in the transfer chamber to be gradually reduced, leading to voids in the transfer chamber and therefore in the cylindrical body 1, resulting in the inflow of air from the discharge port 11 into the transfer chamber. This inflow of air into the transfer chamber decreases the degree of vacuum of the evacuation chamber, and so the efficiency of deaeration of the particles falls leading to a reduction in the increase in bulk density. Thus, the conventional deaerator fails to increase bulk density of particulates substantially. In general, the increase in the bulk density is limited to about 1.3 to 2 times the initial bulk density, although this varies depending upon the type of particulates, the initial bulk density and the like.
Also, the conventional deaerator is generally operated while setting the negative pressure of the vacuum pump within a range between -600 mmHg and -700 mmHg. This tends to cause the filter fabric 13 sandwiched between the perforated metal plates 14' and 14" to stretched or break so that particulates being transferred leak out through the damaged filter fabric to the vacuum pump causing failure of the vacuum pump and the like.
In order to avoid these difficulties, it has been proposed to reduce the porosity of the perforated metal plates 14' and 14" to protect the filter fabric 13 and to compensate for deficiencies in the strength of the filter fabric 13. However, for example, a decrease in the porosity of the perforated metal plates 14' and 14" to about 40% merely leads to a prolongation of the life of the filter by at most about one week. Any further decrease in the porosity causes a decrease in a filtering area of the filter which substantially deteriorates the performance of the deaerator.
The conventional deaerator has another disadvantage in that it is very difficult and troublesome to assemble the filter while accurately aligning the holes of the two perforated metal plates 14' and 14" with one another and so replacement of the filter fabric 13 requires much time and labor.
In the conventional deaerator, deaeration is carried out in such a manner that a cake layer of particulates is formed in the clearance of several millimeters between a screw flight and the filter and the air in the particulates being transferred is removed through the cake layer. Accordingly, when particulates which form such a cake layer of large filtering resistance are treated, it is necessary periodically to carry out removal of the cake layer by backwashing the filter. For this, compressed air of 2-3 kg/cm.sup.2 is generally used and it is often necessary to carry out the backwash operation several times per hour. The filter fabric used for the filter in the conventional deaerator can be damaged substantially by the backwash and this leads to leakage of the particulates through to the vacuum pump. Thus, frequent replacement of the filter fabric is required.
Also, the backwash often causes the fibres constituting the filter fabric to fall off, resulting in the deaerator being unsuitable for use in fields such as food production, medicine production etc. in which it is essential to prevent the inclusion of foreign matter completely.